Transmission protection for communications networks having stations operating with different modulation formats

ABSTRACT

A technique to allow enhanced stations and legacy stations to work with each other without the inefficiencies of signaling overhead in the prior art is disclosed. An enhanced station transmits an initial, short frame using a modulation compatible with legacy stations. The frame sets the duration for a frame exchange—consisting of a data frame, followed by acknowledgement frame—in which the data frame is transmitted using an enhanced modulation format. The duration specified in the transmitted initial frame covers the time interval of the subsequent frame exchange. All stations, including legacy stations, listen in on the frame exchange and refrain subsequently from transmitting spontaneously for the time interval covered by the duration. Alternatively, the frame exchange can comprise multiple data frames with corresponding acknowledgement frames. The enhanced station can also transmit, during the remaining frame exchange, one or more intermediate frames that indicate duration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No.: 60/347,412, entitled “Transmission Protection ForWireless LAN Stations Operating With Different Modulation Formats,”filed on Jan. 12, 2002 and incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to communications protocols in general,and, more particularly, to techniques for reducing the likelihood ofcollisions between data packets in wireless communications channels.

BACKGROUND OF THE INVENTION

The IEEE 802.11 set of protocols includes 802.11(b) and 802.11(a). Alsoknown as 802.11 High Rate or Wi-Fi (i.e., “wireless fidelity”), the802.11(b) approach was approved by the IEEE in 1999 and is currently themainstream technology adopted by wireless device manufacturers.Essentially using a Direct-Sequence Spread Spectrum (DSSS) technique,802.11(b) uses a modulation scheme known as Complementary Code Keying(CCK) to transmit data signals at 11 megabits per second (Mbps) over anunlicensed portion of the radio frequency spectrum at around 2.4 GHz.IEEE 802.11(b) enabled a new generation of products to communicatewirelessly with an Ethernet-like connection. Unfortunately, however, thespeed of 802.11(b) is only one-tenth that of its wired counterpart, IEEE802.3.

The IEEE 802.11(a) standard was approved concurrently with 802.11(b),but utilizes Orthogonal Frequency Division Multiplexing (OFDM) as themodulation technique for signal transmission. OFDM is not compatiblewith 802.11(b) devices because they use CCK modulation. IEEE 802.11(a)technology can transmit data signals at up to 54 Mbps and operates inthe 5 GHz frequency spectrum.

It would be desirable to extend the benefits of higher bit rate OFDMtransmission to the 2.4 GHz band, which, between the two modulations, isthe exclusive domain of the CCK scheme of 802.11(b). The IEEE 802.11(g)standard attempts to merge these operational characteristics together.IEEE 802.11(g) OFDM transmissions, however, are hidden from the legacy802.11(b) nodes, because the 802.11(b) “physical carrier sensemechanism,” explained shortly, does not detect the OFDM carrier.

In the prior art, 802.11(g) nodes can fall back to the “virtual carriersense mechanism” to protect OFDM transmissions from colliding (i.e.,experiencing collisions) with transmissions using other modulations. The802.11 medium access control (MAC) is based around a collision avoidancemechanism, meaning that nodes defer to an active transmission becausethey see that the shared channel (or “medium”) is busy. Their clearchannel assessment is a mechanism that senses a physical carrier on themedium.

Furthermore, the MAC protocol defines a virtual carrier sense mechanism,in addition to the traditional physical carrier sense mechanism. Toimplement the virtual carrier sense mechanism, each node maintains anetwork allocation vector (NAV) counter that indicates whether themedium must be considered busy or not. After each frame reception at anode (whether the frame has been directed to the node or not), the nodeinitializes its NAV counter with a duration value that is obtained fromthe duration field in the frame header of the received frame. Over time,this duration value decrements down until it reaches zero, indicatingthat it is presumptively safe to transmit. Conversely, a non-zero NAVvalue indicates that the virtual carrier sense (and the share channel)is busy.

An acknowledgement (ACK) frame acknowledges receipt of each transmitteddata frame. The ACK frame is NAV protected by the preceding data frame,in which the duration field in the data frame specifies a duration valuethat reserves the medium until the end of the ACK transmission.Alternatively, the first frame transmitted in a signal stream can carrya value in the duration field that covers the entire remaining framesexchanged, possibly comprising multiple data frames and ACK frames. Inother words, the duration value covers the subsequent frame exchange, inwhich each frame exchange is typically one or more pairs of a data frameresponded to with an ACK frame.

The virtual carrier sense mechanism, a familiar part of the 802.11standard, has been previously used to solve a different problem uniqueto wireless networks. First and second nodes can potentially beseparated by a distance greater than their respectively transmittedsignals (carriers) can reach, while an intermediate third node can beclose enough to each of the first and second nodes to hear both signals.

FIG. 1 depicts telecommunications system 100 of the prior art,comprising nodes 102-1, 102-2, and 102-3. Rings 103-1, 103-2, and 103-3represent the respective limits of signal coverage for nodes 102-1,102-2, and 102-3. As depicted, ring 103-1 does not encompass node 102-2,and ring 103-2 does not encompass node 102-1, meaning that the signalsfrom each of the two nodes does not reach the other node. In theexample, the intermediate third node (i.e., node 102-3) is alreadyreceiving from the first node (i.e., node 102-1), and the second node(i.e., node 102-2) has data packets to transmit. The situation can arisethat the second node will not defer its transmission, but instead willalso try to transmit and, in the process, potentially corrupt the activetransmission from the first node. In the example, nodes 102-1 and 102-2are essentially hidden from each other.

If a hidden node case is suspected, then 802.11 nodes can invoke anRTS/CTS mechanism of the prior art before any data transmission,depicted in FIG. 2. This means that prior to sending a data frame, anode transmits, as part of its signal stream 201-1, Request to Send(RTS) frame 202, which contains a duration value that covers interval203 needed for the pending data transmission, including data frame 205and ACK frame 206. RTS frame 202 will set the NAV locally around thesender using this duration value. If the medium is free around thereceiver, it responds, as part of its signal stream 201-2, with Clear toSend (CTS) frame 204, which sets the NAV for all other nodes in thevicinity of the receiver. After the RTS/CTS exchange, other nodes in theareas around the sending and receiving nodes defer their transmissionthrough the virtual carrier sense mechanism.

Although the RTS/CTS mechanism provides interoperability with legacystations, it is suboptimal because it requires the transmission of twoCCK frames (RTS and CTS) prior to the OFDM transmission. The RTS/CTSmechanism is targeted specifically at hidden node situations, in whichthe area at both the sender and the receiver must be NAV protected, eachby a different frame. NAV protection, however, does not necessarily haveto be imposed in all OFDM transmissions, especially where it is knownthat no hidden nodes exist, as shown in the configuration of FIG. 3.

Telecommunications system 300 of the prior art comprises nodes 302-1,302-2, and 302-3, each with a limit of signal coverage represented byrings 303-1, 303-2, and 303-3, respectively. Note that all three nodesare in each of the three areas of signal coverage, signifying that nohidden nodes exist in the configuration. In such a situation where nohidden nodes exist—a property that can be readily determined—it isdisadvantageous to use the additional overhead of the RTS/CTS mechanism.

The need exists for a technique to allow enhanced stations and legacystations to work with each other without the inefficiencies of signalingoverhead in the prior art.

SUMMARY OF THE INVENTION

The present invention provides a technique to allow enhanced stationsand legacy stations to work with each other without the inefficienciesof signaling overhead in the prior art.

In accordance with the illustrative embodiment of the present invention,an enhanced station transmits an initial, short frame using a modulationcompatible with legacy stations. The frame sets the duration for a frameexchange—consisting of a data frame, followed by acknowledgementframe—in which the data frame is transmitted using an enhancedmodulation format. The duration specified in the transmitted initialframe covers the time interval of the subsequent frame exchange. Allstations, including legacy stations, listen in on the frame exchange andrefrain subsequently from transmitting spontaneously for the timeinterval covered by the duration. This protects the frame exchange, evenwhere legacy stations are incapable of listening in on the enhancedmodulation. Alternatively, the frame exchange can comprise multiple dataframes with corresponding acknowledgement frames.

An additional means of providing protection of the frame exchange, inaccordance with another illustrative embodiment of the presentinvention, is by the enhanced station transmitting, during the remainingframe exchange, one or more intermediate frames that indicate duration.The enhanced station transmits the intermediate protection frame orframes using the legacy-compatible modulation. In accordance with avariation of the illustrative embodiment, each intermediate frame canalso carry actual data.

The illustrative embodiment of the present invention comprises:directing to an output queue at a station a data frame to be transmittedover a shared communications network; monitoring at the station for anopportunity to transmit a first signal without colliding with signalspresent on the shared communications network wherein the first signal,when transmitted, is modulated according to a first modulation formatand conveys the data frame; and responsive to identifying an opportunityto transmit without colliding with signals present on the sharedcommunications network, transmitting during the opportunity a secondsignal modulated according to a second modulation format prior totransmitting the first signal, wherein the second signal indicates theduration of the frame exchange of the first signal and correspondingacknowledgement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of the respective coverage areas ofthree communication nodes, hidden nodes present, in the prior art.

FIG. 2 depicts a message flow diagram of transmissions between twocommunication nodes in the prior art.

FIG. 3 depicts a schematic diagram of the respective coverage areas ofthree communication nodes, no hidden nodes present, in the prior art.

FIG. 4 depicts a schematic diagram of the illustrative embodiment of thepresent invention.

FIG. 5 depicts a block diagram of the salient components of station402-x, for x=1 through N, in accordance with the illustrative embodimentof the present invention.

FIG. 6 depicts a message flow diagram of the first variation of thefirst embodiment of the present invention.

FIG. 7 depicts a flowchart of the tasks performed by an enhanced stationin transmitting a frame in the first embodiment of the presentinvention.

FIG. 8 depicts a message flow diagram of the second variation of thefirst embodiment of the present invention.

FIG. 9 depicts a message flow diagram of the first variation of thesecond embodiment of the present invention.

FIG. 10 depicts a message flow diagram of the second variation of thesecond embodiment of the present invention.

FIG. 11 depicts a flowchart of the tasks performed by an enhancedstation in transmitting a frame in the second embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 4 depicts a schematic diagram of the illustrative embodiment of thepresent invention, telecommunications system 400, which transmitssignals between stations (i.e., nodes) 402-1 through 402-N, wherein N isa positive integer, over shared communications network 401. Each ofstations 402-1 through 402-N can be a stationary, portable, or mobiletype with different types in the mix.

In accordance with the illustrative embodiment, telecommunicationssystem 400 is a packet-switched network, in contrast to acircuit-switched network, as is well known to those skilled in the art.In other words, a macro data structure (e.g., a text file, a portion ofa voice conversation, etc.) of indefinite size is not necessarilytransmitted across shared communications network 401 intact, but rathermight be transmitted in small pieces.

Each of these small pieces is encapsulated into a data structure calleda “data frame,” and each data frame traverses shared communicationsnetwork 401 independently of the other data frames. The intendedreceiver of the macro data structure collects all of the data frames asthey are received, recovers the small pieces of data from each, andreassembles them into the macro data structure. This process isdescribed in more detail below.

Shared communications network 401 can be a wireless or wireline orhybrid wireless and wireline network. A salient characteristic of sharedcommunications network 401 is that every data frame transmitted onshared communications network 401 by any station is received or “seen”by every station on shared communications network 401, regardless ofwhether the data frame was intended for it or not. In other words,shared communications network 401 is effectively a broadcast medium.

If shared communications network 401 is wireless, in whole or in part,embodiments of the present invention can use a variety of radio oroptical frequencies and transmission methods. Possible radio frequencyspectrum, if used, includes the Industrial, Scientific, and Medical(ISM) frequency band in the range of 2.4 GHz. Shared communicationsnetwork 401 could be a wireless local area network.

It will be clear to those skilled in the art how to make and use sharedcommunications network 401. It will also be clear to those skilled inthe art that the shared communications network depicted in FIG. 4 isillustrative only and that other types of communications networks arewithin the scope of the present invention.

Stations 402-1 through 402-N receive or generate the macro datastructure and prepare it for transmission over shared communicationsnetwork 401. The macro data structure can represent, for example,telemetry, text, audio, video, etc. Alternatively, one or more ofstations 402-1 through 402-N (e.g., station 402-2, etc.) can function asgateways between shared communications network 401 and othercommunications networks. In functioning as a gateway, a station receivesthe macro data structure from another communications network.

FIG. 5 depicts a block diagram of the salient components of station402-x, for x=1 through N, in accordance with the illustrative embodimentof the present invention. Receiver 501 comprises the wireless orwireline or hybrid wireless and wireline interface circuitry thatenables station 402-x to receive data frames from communications network401. When receiver 501 receives a data frame from shared communicationsnetwork 401, it passes the data frame to processor 502 for processing.It will be clear to those skilled in the art how to make and usereceiver 501.

Processor 502 is a general-purpose or special-purpose processor that iscapable of performing the functionality described below and with respectto FIG. 6 through 10. In particular, processor 502 is capable of storingdata into memory 503, retrieving data from memory 503, and of executingprograms stored in memory 503. Memory 503 accommodates input queues andoutput queues for incoming data and outgoing messages (including dataframes), respectively. It will be clear to those skilled in the art howto make and use processor 502 and memory 503.

Transmitter 504 comprises the wireless or wireline or hybrid wirelessand wireline interface circuitry that enables station 502-x to transmitdata frames onto shared communications network 401. It will be clear tothose skilled in the art how to make and use transmitter 504.

In accordance with the illustrative embodiment of the present invention,not all of stations 402-1 through 402-N are of identical capability.Situations involving stations with heterogeneous capabilities can occur,for example, where modern stations are added to a telecommunicationsystem that comprises only legacy stations. Additionally, the situationcan result where some, but not all, of the stations in atelecommunications system are upgraded with additional capabilities.Whatever the reason, it will be clear to those skilled in the art whytelecommunications systems exist that comprise stations withheterogeneous capabilities.

In accordance with the illustrative embodiment of the present invention,some of stations 402-1 through 402-N are capable of transmission usingan older modulation format, but not a newer modulation format. For thepurposes of this specification, these stations are hereinafter called“legacy stations.” The example of a legacy station in the illustrativeembodiment is an 802.11(b)-capable station using CCK modulation only. Incontrast, others of stations 402-1 through 402-N are capable oftransmission using the newer modulation format, in addition to the oldermodulation format. For the purposes of this specification, thesestations are hereinafter called “upgraded stations.” The example of alegacy station in the illustrative embodiment is an 802.11(g)-capablestation using both OFDM modulation and CCK modulation. In accordancewith the illustrative embodiment of the present invention, legacystations and upgraded stations are capable of communicating with eachother because the upgraded stations transmit data frames that areintended for legacy stations in the modulation format that is used bythe legacy stations.

FIG. 6 depicts a message flow diagram of the first variation of thefirst illustrative embodiment of the present invention. Signal stream601-1 represents the sequence of messages transmitted by a first stationon shared communications network 401, in which at least some of themessages are intended for a second station. Signal stream 601-2represents the sequence of messages transmitted by the second station onshared communications network 401, in which at least some of themessages are intended for the first station. Both stations are of theupgraded type.

Prior to sending a data frame, the first station transmits, as part ofits signal stream 601-1, a frame indicating clear to send, CTS frame602. CTS frame 602 contains a duration field with a value that coverstime interval 603 associated with the frame exchange of pending datatransmission and corresponding acknowledgement. Time interval 603comprises the transmission times for data frame 604 and ACK frame 605.The value of the duration field representing time interval 603 can becalculated, for example, by adding up the anticipated transmission timesof the relevant signals to be subsequently transmitted. The value can bedetermined empirically, it can be estimated, or it can be determined inanother way. It can comprise a margin of variation in transmission, orit can comprise no extra margin. It will be clear to those skilled inthe art how to calculate and set the value of the duration field in CTSframe 602.

It will be clear to those skilled in the art that a different frame canbe used in place of CTS frame 602, such as a null frame, a data framewith an empty payload, etc., to achieve the same purpose of indicatingduration.

As part of the illustrative embodiment, although the first station iscapable of transmitting in an enhanced first modulation format (i.e.,“MOD1”), the first station transmits CTS frame 602 using alegacy-compatible second modulation format (i.e., “MOD2”). This allowslegacy stations to listen in and set their NAV counters to the value ofthe transmitted duration field in CTS frame 602, causing those stationsto refrain from transmitting spontaneously during the duration of theframe exchange. An example of the first modulation format is orthogonalfrequency division multiplexing (OFDM). An example of the secondmodulation format is complementary code keying (CCK). As part of theillustrative embodiment, the first station transmits CTS frame 602 (orequivalent) to itself, consequently not requiring a second station torespond. Furthermore, the first station does not have to acknowledge CTSframe 602, since the node sent the frame to itself, minimizing messageoverhead.

The first station then immediately transmits data frame 604 using theenhanced first modulation format. The second station, upon receivingdata frame 604, responds by transmitting ACK frame 605. ACK frame 605can be sent using either the first modulation format or secondmodulation format, since the first station can understand either format.If ACK frame 605 is sent in the legacy second modulation format, thenadditional protection is added against legacy stations newly arrivinginto shared communications network 401 that were previously unavailableto set their NAV counters. Both data frame 604 and ACK frame 605 areprotected by the NAV counter running in nearby legacy stations.

Throughout the time interval occupied by signal streams 601-1 and 601-2,other stations present on shared communications network 401, comprisinglegacy stations (if present) and other enhanced stations (if present),are presumably monitoring for an opportunity to transmit signals withoutcolliding with signals already present. The legacy stations sense sharedcommunications network 401 for signals modulated according to the secondmodulation format. The stations refrain from transmitting spontaneouslyif a signal is present. Furthermore, the stations refrain fromtransmitting during the time interval specified by the value in thetransmitted duration field.

It will be clear to those skilled in the art how to format, encode,transmit, receive, and decode CTS frame 602 (or equivalent, asdiscussed), data frame 604, and ACK frame 605.

FIG. 7 depicts a flowchart of the tasks constituting the firstillustrative embodiment and performed by an upgraded station in queuingand transmitting a data frame in the presence of legacy stations onshared communications network 401. It will be clear to those skilled inthe art which of the tasks depicted in FIG. 7 can be performedsimultaneously or in a different order than that depicted.

At task 701, the upgraded station directs a formed data frame to anoutput queue. It will be clear to those skilled in the art how to formthe data frame and how to make and use the output queue.

At task 702, the upgraded station monitors for an opportunity totransmit a first signal that conveys the queued data frame, withoutcollision and by using a first modulation format. The first modulationformat can be, for example, the OFDM format existing within an802.11(g)-based wireless local area network. It will be clear to thoseskilled in the art how to recognize when it is improper to transmit andhow to recognize when it is appropriate to transmit. If it is determinedat task 703 that an opportunity exists, control proceeds to task 704.

At task 704, the upgraded station transmits onto shared communicationsnetwork 401 a second signal that is modulated according to a secondmodulation format. As part of the illustrative embodiment, the secondmodulation format is the legacy format understood by all the stations.For example, this can be CCK format, as opposed to the enhanced OFDMformat also existing within an 802.11(g)-based wireless local areanetwork. As part of the illustrative embodiment, the informationconveyed by the second signal indicates the allotted duration ofsubsequently transmitted signals, in this case, the first signaltransmitted by the transmitting station and the correspondingacknowledgement from the receiving station. The value of the durationfield can be calculated, for example, by adding up the anticipatedtransmission times of the relevant signals to be subsequentlytransmitted. The value can be determined empirically, it can beestimated, or it can be determined in another way. It can comprise amargin of variation in transmission, or it can comprise no extra margin.It will be clear to those skilled in the art how to calculate and setthe duration.

The second signal (e.g., conveying a clear to send indication, etc.) istransmitted by the transmitting station to itself (e.g., by the stationspecifying its own address as the destination, etc.). It will be clearto those skilled in the art how a station can transmit a signal toitself.

At task 705, the upgraded station transmits onto shared communicationsnetwork 401 the first signal. The first signal can convey a data frameor it can convey other information. The upgraded station transmits thefirst signal (and can receive a signal indicating an acknowledgement)while under NAV protection as specified in the duration field sentpreviously in the second signal.

It will be clear to those skilled in the art how to perform each oftasks 701 through 705.

FIG. 8 depicts a message flow diagram of the second variation of thefirst illustrative embodiment of the present invention. Signal stream801-1 represents the sequence of messages transmitted by a first stationon shared communications network 401, in which at least some of themessages are intended for a second station. Signal stream 801-2represents the sequence of messages transmitted by the second station onshared communications network 401, in which at least some of themessages are intended for the first station. Both stations are of theupgraded type.

Prior to sending a data frame, the first station transmits, as part ofits signal stream 801-1, a frame indicating clear to send, CTS frame802, which contains a duration field with a value that covers timeinterval 803 associated with the frame exchange of pending datatransmissions and acknowledgements. Time interval 803 comprises thetransmission times for multiple data frames (e.g., data frames 804 and806, etc.) and corresponding ACK frames (e.g., ACK frames 805 and 806,etc.). The value of the duration field representing time interval 803can be calculated, for example, by adding up the anticipatedtransmission times of the relevant signals to be subsequentlytransmitted. The value can be determined empirically, it can beestimated, or it can be determined in another way. It can comprise amargin of variation in transmission, or it can comprise no extra margin.It will be clear to those skilled in the art how to calculate and setthe value of the duration field in CTS frame 802.

It will be clear to those skilled in the art that a different frame canbe used in place of CTS frame 802, such as a null frame, a data framewith an empty payload, etc., to achieve the same purpose of indicatingduration. As part of the illustrative embodiment, although the firststation is capable of transmitting in an enhanced first modulationformat (i.e., “MOD1”), the first station transmits CTS frame 802 insimilar fashion as is CTS frame 602 and for similar reasons.

The first station then immediately transmits first data frame 804 usingthe enhanced first modulation format. The second station, upon receivingfirst data frame 804, responds by transmitting ACK frame 805. ACK frame805 can be sent using either the first modulation format or secondmodulation format, since the first station can understand either format.If ACK frame 805 is sent in the legacy second modulation format, thenadditional protection is added against legacy stations newly arrivinginto shared communications network 401 that were previously unavailableto set their NAV counters. Both first data frame 804 and ACK frame 805are protected by the NAV counter running in nearby legacy stations.

The first station can then subsequently transmit additional data frames,paired with additional ACK frames sent by the second station. Finally,the first station transmits last data frame 806 using the enhanced firstmodulation format. The second station, upon receiving last data frame806, responds by transmitting ACK frame 807. ACK frame 807 is sent insimilar fashion as ACK frame 805. Both last data frame 806 and ACK frame807 are protected by the NAV counter running in nearby legacy stations.

Throughout the time interval occupied by signal streams 801-1 and 801-2,other stations present on shared communications network 401, comprisinglegacy stations (if present) and other enhanced stations (if present),are presumably monitoring for an opportunity to transmit signals withoutcolliding with signals already present. The legacy stations sense sharedcommunications network 401 for signals modulated according to the secondmodulation format. The stations refrain from transmitting spontaneouslyif a signal is present. Furthermore, the stations refrain fromtransmitting during the time interval specified by the value in thetransmitted duration field.

It will be clear to those skilled in the art how to format, encode,transmit, receive, and decode CTS frame 802 (or equivalent, asdiscussed), first data frame 804, last data frame 806, and ACK frames805 and 807.

FIG. 9 depicts a message flow diagram of the first variation of thesecond illustrative embodiment of the present invention. Signal stream901-1 represents the sequence of messages transmitted by a first stationon shared communications network 401, in which at least some of themessages are intended for a second station. Signal stream 901-2represents the sequence of messages transmitted by the second station onshared communications network 401, in which at least some of themessages are intended for the first station. Both stations are of theupgraded type.

Prior to sending a data frame, the first station transmits, as part ofits signal stream 901-1, a frame indicating clear to send, CTS frame902. CTS frame 902 contains a duration field with a value that coverstime interval 903 associated with the frame exchange of pending datatransmissions and acknowledgements. Time interval 903 comprises thetransmission times for multiple data frames (e.g., data frames 904, 906,and 910; etc.) and corresponding ACK frames (e.g., ACK frames 905, 907,and 911; etc.). The value of the duration field representing timeinterval 903 can be calculated, for example, by adding up theanticipated transmission times of the relevant signals to besubsequently transmitted. The value can be determined empirically, itcan be estimated, or it can be determined in another way. It cancomprise a margin of variation in transmission, or it can comprise noextra margin. It will be clear to those skilled in the art how tocalculate and set the value of the duration field in CTS frame 902.

It will be clear to those skilled in the art that a different frame canbe used in place of CTS frame 902, such as a null frame, a data framewith an empty payload, etc., to achieve the same purpose of indicatingduration. As part of the illustrative embodiment, although the firststation is capable of transmitting in an enhanced first modulationformat (i.e., “MOD1”), the first station transmits CTS frame 902 insimilar fashion as CTS frame 602 and for similar reasons.

The first station can then immediately transmit first data frame 904using the enhanced first modulation format. The second station, uponreceiving first data frame 904, responds by transmitting ACK frame 905.ACK frame 905 can be sent using either the first modulation format orsecond modulation format, since the first station can understand eitherformat. If ACK frame 905 is sent in the legacy second modulation format,then additional protection is added against legacy stations newlyarriving into shared communications network 401 that were previouslyunavailable to set their NAV counters. Both first data frame 904 and ACKframe 905 are protected by the NAV counter running in nearby legacystations.

The first station can then subsequently transmit additional data frames(e.g., data frame 906, etc.), paired with additional ACK frames (e.g.,ACK frame 907, etc.) sent by the second station.

At some point interposed in the series of data frame transmissions, thefirst station can choose to transmit an intermediate, reinforcingprotection frame. Specifically, the first station transmits, as part ofits signal stream 901-1, null frame 908, which contains a duration fieldwith a value that covers time interval 909 associated with the frameexchange of pending data transmissions and acknowledgements that remain.Time interval 909 comprises the transmission times for multiple dataframes (e.g., data frame 910, etc.) and corresponding ACK frames (e.g.,ACK frame 911, etc.). It will be clear to those skilled in the art howto calculate and set the value of the duration field in null frame 908.

It will be clear to those skilled in the art that a different frame canbe used in place of null frame 908 to achieve the same purpose ofindicating duration. As part of the illustrative embodiment, althoughthe first station is capable of transmitting in an enhanced firstmodulation format (i.e., “MOD1”), the first station transmits null frame908 in similar fashion as CTS frame 602 and for similar reasons. Thefirst station can transmit null frame 908 intermittently whenever it isdetermined to do so. The transmission can be based upon time, newstations arriving into shared communications network 401, etc. If basedupon time, the transmission can be periodic or aperiodic. It will beclear to those skilled in the art how to determine when null frame 908(or equivalent, as discussed) is transmitted.

The first station transmits data frame 910 using the enhanced firstmodulation format. The second station, upon receiving data frame 910,responds by transmitting ACK frame 911. ACK frame 911 is sent in similarfashion as ACK frame 905. The NAV counter running in nearby legacystations protects both data frame 910 and ACK frame 911, in addition toany additional data frame/ACK frame pairs transmitted during theduration period.

Throughout the time interval occupied by signal streams 901-1 and 901-2,other stations present on shared communications network 401, comprisinglegacy stations (if present) and other enhanced stations (if present),are presumably monitoring for an opportunity to transmit signals withoutcolliding with signals already present. The legacy stations sense sharedcommunications network 401 for signals modulated according to the secondmodulation format. The stations refrain from transmitting spontaneouslyif a signal is present. Furthermore, the stations refrain fromtransmitting during the time interval specified by the value in thetransmitted duration field.

It will be clear to those skilled in the art how to format, encode,transmit, receive, and decode CTS frame 902 (or equivalent, asdiscussed); null frame 908 (or equivalent, as discussed); data frames904, 906, and 910; and ACK frames 905, 907, and 911. Finally, it will beclear to those skilled in the art that multiple intermediate frames(e.g., null frame 908, etc.) can be transmitted to reinforce the NAVprotection.

FIG. 10 depicts the second variation of the second illustrative of thepresent invention. Signal stream 1001-1 as transmitted by a firststation comprises CTS frame 1002, and data frames 1004, 1006, 1008, and1011. Signal stream 1001 as transmitted by a second station comprisesACK frames 1005, 1007, 1010, and 1012. The variation depicted is similarto that depicted in FIG. 9, except that a data frame (i.e., data frame1008) is used to reinforce the NAV protection, instead of a null frame.

Specifically, the first station transmits, as part of its signal stream1001-1 and at an intermediate point, data frame 1008, which contains aduration field with a value that covers time interval 1009 associatedwith the frame exchange of pending data transmissions andacknowledgements that remain. Time interval 1009 comprises thetransmission times for multiple data frames (e.g., data frame 1011,etc.) and corresponding ACK frames (e.g., ACK frames 1010 and 1012,etc.). It will be clear to those skilled in the art how to calculate andset the value of the duration field in data frame 1008.

As part of the illustrative embodiment, although the first station iscapable of transmitting in an enhanced first modulation format (i.e.,“MOD1”), the first station transmits data frame 1008 using alegacy-compatible second modulation format (i.e., “MOD2”). This allowslegacy stations to listen in and set their NAV counters to the value ofthe transmitted duration field in data frame 1008, causing thosestations to refrain from transmitting spontaneously during the durationof the frame exchange. Note that data frame 1008 also contains validdata that had to be transmitted in some data frame.

Throughout the time interval occupied by signal streams 1001-1 and1001-2, other stations present on shared communications network 401,comprising legacy stations (if present) and other enhanced stations (ifpresent), are presumably monitoring for an opportunity to transmitsignals without colliding with signals already present. The legacystations sense shared communications network 401 for signals modulatedaccording to the second modulation format. The stations refrain fromtransmitting spontaneously if a signal is present. Furthermore, thestations refrain from transmitting during the time interval specified bythe value in the transmitted duration field.

It will be clear to those skilled in the art how to format, encode,transmit, receive, and decode CTS frame 1002 (or equivalent, asdiscussed); data frames 1004, 1006, 1008, and 1011; and ACK frames 1005,1007, 1010, and 1012. Finally, it will be clear to those skilled in theart that multiple intermediate frames (e.g., data frame 1008, etc.) canbe transmitted to reinforce the NAV protection.

FIG. 11 depicts a flowchart of the tasks constituting the secondillustrative embodiment and performed by an upgraded station in queuingand transmitting a data frame in the presence of legacy stations onshared communications network 401. It will be clear to those skilled inthe art which of the tasks depicted in FIG. 11 can be performedsimultaneously or in a different order than that depicted.

At task 1101, the upgraded station directs formed data frames to anoutput queue. It will be clear to those skilled in the art how to formdata frames and how to make and use the output queue.

At task 1102, the upgraded station monitors for an opportunity totransmit a first signal that conveys the queued data frames, withoutcollision and by using a first modulation format. The first modulationformat can be, for example, the OFDM format existing within an802.11(g)-based wireless local area network. It will be clear to thoseskilled in the art how to recognize when it is improper to transmit andhow to recognize when it is appropriate to transmit. If it is determinedat task 1103 that an opportunity exists, control proceeds to task 1104.

At task 1104, the upgraded station transmits onto shared communicationsnetwork 401 a second signal that is modulated according to a secondmodulation format. As part of the illustrative embodiment, the secondmodulation format is the legacy format understood by all the stations.For example, this can be CCK format, as opposed to the enhanced OFDMformat also existing within an 802.11(g)-based wireless local areanetwork. As part of the illustrative embodiment, the informationconveyed by the second signal indicates the allotted duration ofsubsequently transmitted signals, in this case, the first signalcomprising a plurality of data frames, the third signal comprisingintermediate protection frames, and the corresponding acknowledgementsfrom the receiving station. The value of the duration field can becalculated, for example, by adding up the anticipated transmission timesof the relevant signals to be subsequently transmitted. The value can bedetermined empirically, it can be estimated, or it can be determined inanother way. It can comprise a margin of variation in transmission, orit can comprise no extra margin. It will be clear to those skilled inthe art how to calculate and set the duration.

The second signal (e.g., conveying a clear to send indication, etc.) istransmitted by the transmitting station to itself (e.g., by the stationspecifying its own address as the destination, etc.). It will be clearto those skilled in the art how a station can transmit a signal toitself.

At task 1105, the upgraded station transmits onto shared communicationsnetwork 401 a portion of the first signal. The portion of the firstsignal can convey a single data frame or it can convey otherinformation. The upgraded station transmits the portion of the firstsignal (and can receive a signal indicating an acknowledgement) whileunder NAV protection as specified in the duration field sent previouslyin the second signal.

At task 1106, if additional frames of any kind are to be transmitted,control proceeds to task 1107. If not, execution of the tasks depictedin FIG. 11 stops.

At task 1107, if it is time to transmit a third signal, control proceedsto task 1108. If not, control proceeds to task 1105.

At task 1108, the upgraded station transmits onto shared communicationsnetwork 401 a third signal that is modulated according to the secondmodulation format, which, as explained earlier, is the legacy formatunderstood by all the stations. For example, this can be CCK format, asopposed to the enhanced OFDM format also existing within an802.11(g)-based wireless local area network. As part of the illustrativeembodiment, the information conveyed by the third signal indicates theallotted duration of subsequently transmitted signals, in this case, theremaining portions of the first signal comprising one or more dataframes, any remaining third signals comprising intermediate protectionframes, and the corresponding one or more acknowledgements from thereceiving station. The duration represented in the third signal can becalculated by adding up the anticipated transmission times of therelevant signals to be subsequently transmitted. It will be clear tothose skilled in the art how to calculate and set the duration.

The third signal, if conveying a null frame or similar non-data framemessage, is transmitted by the transmitting station to itself (e.g., bythe station specifying its own address as the destination, etc.). Itwill be clear to those skilled in the art how a station can transmit asignal to itself. Alternatively, the third signal can convey a dataframe carrying underlying data that would had to have been transmittedto another station, anyway. In this alternative case, the third signalis actually intended for a receiving station, although all stations areable to listen to it (since it is transmitted in the second modulationformat) and read the duration field information.

The first station can transmit the third signal intermittently wheneverit is determined to do so. The transmission can be based upon time, newstations arriving into shared communications network 401, etc. If basedupon time, the transmission can be periodic or aperiodic. It will beclear to those skilled in the art how to determine when the third signalis transmitted.

It will be clear to those skilled in the art how to perform each oftasks 1101 through 1108.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. It is thereforeintended that such variations be included within the scope of thefollowing claims and their equivalents.

1. A method comprising: (a) monitoring a shared-communications mediumfor an opportunity to transmit a first signal and a second signal; (b)transmitting said second signal in accordance with a second modulationscheme on said shared-communications medium, wherein: (i) said secondsignal conveys a frame indicating clear to send that is addressed to thesender of said frame indicating clear to send; and (ii) said frameindicating clear to send comprises a duration field that has a valuebased on the expected length of time required to transmit at least onedata frame; and (c) transmitting said first signal in accordance with afirst modulation scheme on said shared-communications medium after saidsecond signal, wherein said first signal conveys said at least one dataframe; wherein said frame indicating clear to send and said at least onedata frame are addressed to different stations.
 2. The method of claim1, wherein said second modulation scheme is different than said firstmodulation scheme.
 3. The method of claim 1, wherein said firstmodulation scheme comprises orthogonal frequency division multiplexingand said second modulation scheme comprises complementary code keying.4. The method of claim 1, wherein said frame indicating clear to send isat least one of a clear-to-send frame and a frame with an empty payload.5. The method of claim 1, further comprising transmitting a third signalon said shared-communications medium, wherein said third signal conveysa data frame in accordance with said second modulation scheme.
 6. Themethod of claim 1, wherein said shared-communications medium isoperative in the 2.4 GHz Industrial, Scientific, Medical band of theradio frequency spectrum.
 7. A station comprising: (a) a receiver formonitoring a shared-communications medium for an opportunity to transmita first signal and a second signal; and (b) a transmitter for: (1)transmitting said second signal in accordance with a second modulationscheme on said shared-communications medium, wherein: (i) said secondsignal conveys a frame indicating clear to send that is addressed to thesender of said frame indicating clear to send; and (ii) said frameindicating clear to send comprises a duration field that has a valuebased on the expected length of time required to transmit at least onedata frame; and (2) transmitting said first signal in accordance with afirst modulation scheme on said shared-communications medium after saidsecond signal, wherein said first signal conveys said at least one dataframe; wherein said frame indicating clear to send and said at least onedata frame are addressed to different stations.
 8. The station of claim7, wherein said second modulation scheme is different than said firstmodulation scheme.
 9. The station of claim 7, wherein said firstmodulation scheme comprises orthogonal frequency division multiplexingand said second modulation scheme comprises complementary code keying.10. The station of claim 7, wherein said frame indicating clear to sendis at least one of a clear-to-send frame and a frame with an emptypayload.
 11. The station of claim 7, wherein said transmitter alsotransmits a third signal on said shared-communications medium, conveyinga data frame in accordance with said second modulation scheme.
 12. Thestation of claim 7, wherein said shared-communications medium isoperative in the 2.4 GHz Industrial, Scientific, Medical band of theradio frequency spectrum.
 13. A method comprising: (a) monitoring ashared-communications medium for an opportunity to transmit a firstsignal, a second signal, and a third signal; (b) transmitting saidsecond signal in accordance with a second modulation scheme on saidshared-communications medium, wherein: (i) said second signal conveys aframe indicating clear to send that is addressed to the sender of saidframe indicating clear to send; and (ii) said frame indicating clear tosend comprises a duration field that has a value based on the expectedlength of time required to transmit at least one data frame; (c)transmitting said first signal in accordance with a first modulationscheme on said shared-communications medium after said second signal,wherein said first signal conveys said at least one data frame; and (d)transmitting said third signal in accordance with said second modulationscheme on said shared-communications medium after said first signal,wherein said third signal conveys a data frame; wherein said frameindicating clear to send and said at least one data frame are addressedto different stations.
 14. The method of claim 13, wherein said secondmodulation scheme is different than said first modulation scheme. 15.The method of claim 13, wherein said first modulation scheme comprisesorthogonal frequency division multiplexing and said second modulationscheme comprises complementary code keying.
 16. The method of claim 13,wherein said frame indicating clear to send is at least one of aclear-to-send frame and a frame with an empty payload.
 17. The method ofclaim 13, wherein said frame indicating clear to send and said dataframe are addressed to different stations.
 18. The method of claim 13,wherein said shared-communications medium is operative in the 2.4 GHzIndustrial, Scientific, Medical band of the radio frequency spectrum.19. A station comprising: (a) a receiver for monitoring ashared-communications medium for an opportunity to transmit a firstsignal, a second signal, and a third signal; and (b) a transmitter for:(1) transmitting said second signal in accordance with a secondmodulation scheme on said shared-communications medium, wherein: (i)said second signal conveys a frame indicating clear to send that isaddressed to the sender of said frame indicating clear to send; and (ii)said frame indicating clear to send comprises a duration field that hasa value based on the expected length of time required to transmit thesubsequent data frames conveyed by said first signal and said thirdsignal; (2) transmitting said first signal in accordance with a firstmodulation scheme on said shared-communications medium after said secondsignal, wherein said first signal conveys said at least one data frame;and (3) transmitting said third signal in accordance with said secondmodulation scheme on said shared-communications medium after said firstsignal, wherein said third signal conveys a data frame; wherein saidframe indicating clear to send and said at least one data frame areaddressed to different stations.
 20. The station of claim 19, whereinsaid second modulation scheme is different than said first modulationscheme.
 21. The station of claim 19, wherein said first modulationscheme comprises orthogonal frequency division multiplexing and saidsecond modulation scheme comprises complementary code keying.
 22. Thestation of claim 19, wherein said frame indicating clear to send is atleast one of a clear-to-send frame and a frame with an empty payload.23. The station of claim 19, wherein said transmitter also transmits thethird signal on said shared-communications medium, conveying a dataframe in accordance with said second modulation scheme.
 24. The stationof claim 19, wherein said shared-communications medium is operative inthe 2.4 GHz Industrial, Scientific, Medical band of the radio frequencyspectrum.